1. Field of the Invention
The present invention relates to a friction member used in a vibration wave driving apparatus which drives using a vibration wave, and a device using the vibration wave driving apparatus as a driving source.
2. Related Background Art
The following schematically describes the principle of a vibration wave motor which is a typical example of the vibration wave driving apparatus making use of a traveling vibration wave. A vibration member (stator) is constructed in such structure that two groups of piezoelectric elements arranged in the circumferential direction are fixed to one surface of a ring elastic member of an elastic material whose total length is an integral multiple of a certain length .lambda.. These piezoelectric elements are arranged at the pitch of .lambda./2 and in alternately inverse expansive polarities in each group. The piezoelectric elements are arranged with a shift equal to an odd multiple of .lambda./4 between the two groups. The two groups of piezoelectric elements are provided each with an electrode film. When an alternating voltage is applied to only either one group (hereinafter referred to as phase A), the above vibration member generates throughout the entire circumference thereof a standing wave (of the wavelength .lambda.) of such bending vibration that antinodes are located at center points of the respective piezoelectric elements in the group and at points every .lambda./2 apart therefrom and that nodes are located at center points between the positions of the antinodes. When the alternating voltage is applied to only the other group (hereinafter referred to as phase B), the standing wave also appears similarly, though the positions of the antinodes and nodes are shifted by .lambda./4 from those of the standing wave in phase A.
When alternating voltages having an equal frequency and a temporal phase difference of 90.degree. between them are applied to the respective phases A, B, the two standing waves are combined, so that a traveling wave (of the wavelength .lambda.) of such bending vibration as to vibrate in the circumferential direction appears in the vibration member. At this time, each point in the above vibration member having the thickness is in elliptic motion. If a movement member (rotor), for example a ring movement member, is kept in press contact with one surface of the vibration member, the movement member will be subject to frictional force along circumferential direction from the vibration member, so as to be rotated. It is verified that when a plurality of radial grooves are formed in the circumferential direction on the other side than the fixing surface of the piezoelectric elements in the vibration member in order to increase the circumferential component of the elliptic motion, the neutral plane of vibration is shifted toward the fixing surface of piezoelectric elements, the rotational frequency increases even without change in the amplitude, and the effect of also increasing motor efficiency is great. The grooves also present the effect of removing abrasion powder.
On the basis of the above principle, the vibration wave motor has the following advantages:
1) it has holding torque during no power supply and does not bring about hunting during positioning operation;
2) a rise and a fall of rotation are quick (i.e., a mechanical time constant is small), because inertia is small and driving torque is large;
3) it is free of cogging, because equal driving force is generated at all points on the circumference; and so on.
It can be mentioned from these advantages that the vibration wave motor is suitable for high precision rotation and high precision positioning in terms of the principle, but it has the disadvantage of change in motor performance according to change of the friction surfaces with time because of abrasion of the friction portions. A variety of friction materials of composite resins have been proposed heretofore in order to compensate for the disadvantage. Among them, polytetrafluoroethylene (hereinafter referred to as PTFE) resins filled with various fillers have been proposed as materials having both stability of friction performance and durability, as disclosed in Japanese Patent Applications Laid-Open Nos. 1-206880, 2-285974, 5-184168, and so on.
Many PTFE resins are commonly used as friction materials, without being limited only to the vibration wave motors. While the PTFE resins are kept sliding, they suffer laminar exfoliation, because they are materials having low surface energy from the molecular aspect. Since such materials suffering laminar exfoliation have the action of stabilizing coefficients of friction in low levels, they are often used as additives for the friction materials, for example, like graphite, molybdenum disulfide, and mica. When PTFE among them is used as an additive, it is unlikely to damage a counterpart because of its low elastic modulus. In addition, an elongation characteristic of the PTFE is great, because it is a polymer material. Therefore, it is easy to transfer to the counterpart and it is considered that friction occurs between the PTFE transferred to the counterpart during friction (which is generally called "transfer film") and PTFE, thus accomplishing a stable friction state with a low coefficient of friction.
However, since PTFE is apt to suffer the laminar exfoliation under friction without any additive, it has the disadvantages of great abrasion and being prone to creep because of its low elastic modulus. For compensating for the disadvantages, there are two techniques employed.
The first technique is a method of adding the PTFE to another strong resin (for example, thermosetting or thermoplastic polyimide, polyamideimide, polyetherimide, polyether ether ketone, polyether sulfone, etc.). However, this technique often fails to achieve the stable friction state because of great influence of the matrix resin. When the vibration wave motor was actually constructed using these composite resins as friction materials, abrasion of the matrix resin was heavy and the abrasion powder was jammed in and stuck to the friction portions. This made output very unstable. Therefore, it is difficult to use these composite resins well.
The other technique is a method of using PTFE as a matrix and adding various fillers thereto in order to improve the creep characteristic and abrasion resistance. For example, materials commercially available are PTFE resins containing glass fiber, carbon fiber, aromatic polyester powder, and polyimide powder. In the case of the PTFE resins with only either of polyimide, aromatic polyester, etc., coefficients of friction sometimes decrease with driving time, so as to deteriorate the torque performance of the vibration wave motor in certain cases. In the case of the composite resins with only either of carbon fiber, glass fiber, etc., abrasion is sometimes great and it is also conceivable that the motor performance could become instable, including variation in the torque performance, occurrence of noise, and so on.